Multistage audio amplification system



Sept. 13, I932. E. E. HILER: 1,876,674

MUL'I'ISTAGE AUDIO AMPLIFICATION SYSTEM Filed Aug. 21, 1929 filament orground portion of the'other terminals of thls Wll'ldlllg. are con-.

Patented Sept. 13, 1932 arsen EDWARD E. trunn on BLOOMFIELD, NEW JERSEY,

' MULTIS'TAGE AUDIO AMPLfiiiGATiON SYSTEM Application'filed August21,1929. Serial1o.387,318.

This invention relates, generally, to electron discharge deviceamplification systems; and the invention has reference, moreparticularly, to multi-stage push-pull audio frec quency amplifiercircuit-s. I 1

'Heretofore, use has beenmade of transformer coupled push-pullaudioamplification, wherein the secondaryiwinding ofthe inputtransformer is center tapped .to the the circuit and mated to the gridsof the oppositely arranged electron discharge devices. This arrangementeliminates the elfect of the second harmonics generated in the electrondischarge devices,.for in operation at any given instant the voltageimpressed on the grid of one device is increasing-while that impressedon the 1 grid of the other device is decreasing. Thus the grid or inputcircuits of the devices are 180 degrees out of phase causingthefundamental currents in the two pla-te circuitsto differ'by. 180degrees. A center tapped. coil may be used in the output circuit of the,elec- 7 tron discharge devices in lieu of an output transformer, 1nwhlch case the center tap of the coil is connected to the positiveterminal of a- B battery, and the other terminals are connected to theplates ofthe two devices.

lVhen employing the center tapped coil, the fundamental signal inpassing from the plate of one device or tube through the output" coil tothe center tap is 180 degrees out of phase with the fundamental signalpassing from the plate of the other device to the said center tap.However, owing to the push-pull arrangement, the fundamental cur rentspassing from the plates of the-devices through the sections of the coilare in phase, causing their magnetic fields to be additive, as desired,while the harmonics are balanced out.

' The use of audio transformers in the input circuits of suchamplification systems, however, results in distortion due to theimpossibility of obtaining one hundred percent coupling between theprimary winding and the tapped secondary winding of the transformer. 7,Furthermore, in using input transformers in such circuits it isgenerally necessary in order to obtain satisfactoryoperation to employresistances inshunt with the secondarle's of the transformers, which resstances have the effect of reducing the undesirable effects of resonancepeaks, the use of-which resistances-,however, not only complicatesthe'circuit but adds to the cost there-- of. When more than one stage oftransformer coupled push-pullaudio amplification is employed, thedistortion increases.

In my copending application Serial No.

3T8,058,filed July 13, 1929, there is disclosed a novel-form ofsinglestage push-pull audio frequency amplifier circuit which eliminatesthe objections'a'nd undesirable features present in the use oftransformer coupled push pull audio amplifiers. The principal object ofthe present invention is to provide a novel-multi-stage pushpull audiofrequency amplifier circuit emi ploying double impedance coupling inlieu of transformer coupling. r

The use of the noveldouble impedance coupling of this invention not onlyproduces substantially distortionless amplification but 7 has otherdecided advantages over transformer coupling; One of these advantageslies in the elimination of shunt resistances, since capacity coupling isemployed instead of magnetic coupling, which design eliminates theresonant peaks at the high'frequene cies. On account of not having acutofi at the highfrequencies, the range of frequencies made possible byusing double impedance coupling can be extendedto cover the wide band offrequenciesnecessary for reproduction in high quality music andtelevision applications. V V r I By utilizing the method outlined in 'myprior Patent No. 1,715,501,the low audio frequencies below 150cycles canbe exagger ated, if desired, to compensate for the usually poor responseof loud speakers to low frequencles. Y

7 Other objects of thisinvention not at this time more particularlyenumeratediwill be clearly understood from the. following detaileddescription of the same. e The invention is clearly illustrated in theaccompanying drawing, 'the sin'gle figure of which is a wiring diagramof a multi-stage push-pull audio frequency amplification systemembodying the principles of this invention.

Referring now to the said figure, the reference character 1 designatesan electron discharge device or tube which may be employed as a detectoror as an audio amplification tube. The grid 2 and filament heatedcathode 3 of the tube 1 may be connected to any suitable input circuit,the output of which it is desired to amplify. The heating filament 4 oftube 1 may be heated by alternating current supplied by a generator 5,the

said generator being coupled in any suitable manner to the filament 4,as by a transformer 6. A resistance 7 is connected across the terminalsof the secondary winding of transformer 6, and the mid-point ofresistance 7 is connected to the lead of cathode 3 which lead isillustrated as grounded at 8. This arrangement causes the potential ofthe cathode 3 to be the same as the average potential of the heatingfilament 4. The plate or anode 10 of this tube is illustrated asconnected'through a plate choke coil 11 to a terminal post 12 adapted tobe connected to a B eliminator terminal. Inasmuch as any suitable Beliminator may be employed for this purpose and as the same forms nopart of the present invention it is not shown in the drawing.

The plate 10 of tube 1 is connected through a coupling condenser-14 tothe grid of a tube 15 arranged in opposition or push-pull relation witha similar tube 16. Generator 5 is illustrated as supplying heatingcurrent to the cathode heating filaments of tubes 15 and 16. Theexternal cathode-grid circuit of tube 15 extends through a resistance 18and the upper half of a center tapped grid choke coil 19. The resistance18 serves to cause the desired negative bias to be placed upon thegrid'of tubes 15 and 16. Acondenser 17 is connected across theresistance-18 for bypassing alternating currents. The center tap ofchoke coil 19 is grounded at 20. The external filament-grid circuit oftube 16 extends through resistance 18 and the lower half of grid chokecoil 19. The upper half of grid choke coil 19 serves not onlyas agrid'choke for tube 15 but also as the primary of a one to onetransformer, the secondary of which is the lower half of choke coil 19.Grid choke coil 19 therefor serves to cause the potentials impressed onthe grids oftubes 15 and 16 to be substantially 180 degrees out of phaseso that the fundamental plate circuit currents of these tubes alsodiffer in phase'by 180 degrees while the harmonics generated will be inphase.

The plates or anodes of tubes 15 and 16 are connected to the outerterminals of a center tapped plate choke coil 22. The center of chokecoil 22 is connected to a terminal post 21 adapted to be connected inturn to a suitable B eliminator terminal. Owing to the push-pullarrangement of tubes 15 and 16, though the fundamental currents in theplate circuits of these tubes differ by 180 degrees, yet the magneticfields set up by these currents in the choke coil 22 are additive whilethose set up by the harmonics are opposed and olfset each other.

The plate of tube 15 is connected through a coupling condenser 23 to thegrid of a tube 24 arranged in opposition or push-pull relation with asimilar tube 25. The plate of tube 16 is connected through a couplingcondenser 26 to the grid of tube 25. A center tapped grid choke coil 27is connected between the grid input circuits of tubes 24 and 25 and hasits center tap grounded at 29. The external cathode-grid circuit of tube24 extends through a resistance 28 and the upper half of grid choke coil27, whereas the external cathode-grid circuit of tube 25 extends throughresistance 28 and the lower half of grid choke coil 27. A condenser 30is connected across resistance 28. The cathode heating filaments oftub-es 24 and 25 are also shown as supplied with heating current fromthe generator 5.

The plates or anodes of tubes 24 and 25 are connected to the outerterminals of a center tapped plate choke coil 31. The center tap ofchoke coil 31 is connected to the terminal post 32 provided forconnection to a B eliminator. Owing to the push-pull arrangement oftubes 24 and 25, though the fundamental currents in the plate circuitsof these tubes differ by 180 degrees, yet the magnetic fields set up bythese currents in the choke coil 31 are additive while those set up bythe harmonics are opposed and offset each other.

The plate or anode of tube 24 is connected through a coupling condenser34 to the grid of a three electrode tube 35 arranged in opposition orpush-pull relation with a similar tube 36. The plate of tube 25 isconnected through a coupling condenser 37 to the grid of tube 36. Acenter tapped grid choke coil 38 is connected between the grid inputcircuits of tubes 35 and 36 and has its center tap grounded at 39.

The electron discharge devices 35 and 36 are illustrated as havingfilamentary cathodes which are shown as heated by alternating currentsupplied by a generator 41 through a transformer 42. A resistance 43 isconnected across the terminals of the secondary winding of transformer42, and the mid-point of resistance 43 is connected through a gridbiasing resistance 44 to ground 45. A condenser 46 is connected acrossresistance 44 for by-passing alternating currents.

The external filament-grid circuit of the tube 35 extends throughresistance 44,

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grounds 45 and 39 andthe upper half of grid choke coil '38,'wh-ereas theexternal filament-gridcircuit of tube 36 extends throu h resistance 44,grounds 45 and 39 and t e lower half of grid choke coil 38.

The plates or anodes of tubes'35 and 36 are connected to the outerterminals of a center tapped output choke coil 47 The center tap ofchoke coil 47 is illustrated as connects to a terminal post 48 providedfor connection' to a B eliminator. "A loud speaker or telephone 49 isillustrated as connected in shunt-across the terminals of choke coil 47.

In opera'tiomthe output of tube 1 impresses an alternating currentpotential across choke coil 11. Since condenser '14 and choke coil19'are connected in shunt with choke coil 11 through the 'B eliminator,an alternating current potential of substantially the same valueas'impressed across coil 11' is impressed upon choke coil 19 since thereis but little drop in potential across condenser 14. Analternatingcurrent potential is therefore applied to the grid ofelectron discharge device 15. Since the lower half of coil 19 ismagnetically coupled to the upper half thereof, an alternating currentpotential is impressed on the grid of tube 16 whichpotential, however,increases simultaneously with a decrease in potential impressed on thegrid of tube 15.

Assuming at any instant that the grid potential of tube causes thefundamental plate A. G. of this tube to flow from the center tap of coil22 in a direction toward the plate of tube 15, then there will be causedby the grid potential of tube 16 a plate current flowing in a directionfrom the plate of tube 16 toward the center tap of coil 22, so thatthese currents are additive in these coils. the art, the voltagesimpressed on the grids of these tubes generate harmonics which are inphase and which therefor balance out in the plate circuits.

Since condenser 23 and the upper half of choke coil 27 are connected inshunt with the upper half of choke coil 22 through the B eliminator, analternating current potential of substantially the same value asimpressed across the upper half of coil 22 is impressed upon the upperhalf of coil 27 since there is but little drop in potential acrosscondenser 28. An alternative current potential is therefore applied tothe grid of electron discharge device 24. Also, since condenser 26 andthe lower half of choke coil 27 are connected in shunt with the lowerhalf of coil 22 through the B eliminator, an alternating currentpotential of substantially the same value as impressed across the lowerhalf of coil 22 is impressed upon the lower half of coil 27 causing analternating current potential to be applied to the grid of electrondischarge device 25.

As is well known to those skilled in Since thefAlC; potential in theplateseib was of devices 15 and. lfi areisubstantially 180 degrees outofphasethepotentials'iin pressed on the gridsof devices and will also besubstantially 180 degrees out'of phase causing the platecurrents ofthese tubes to be likewis'ejout' of phase but never theless additivein-their magnetic effect-in d the-choke coil 31..

The fundamental signals I delivered hy tubes 15 andldandr as amplifiedby tube's 24 and 25 is delivered to tubes -and 36 for furtheramplification. Obviously "as many stages ofaudio amplification asdesired-may be employed for "amplifying the fundamental signalsTheoutput currents of tubes 35 and 36areilikewiseadditive in the outputchoke coil 47 while harmonicsare cancelled Out. '.'l f

Since extraneous B eliminator'A. G.-volt.- ages, which would tend toproduce a hum at the speaker 49, are delivered to the cen-te'r taps ofcoils 22, 31 and 47.-and divide flowing in opposite directions throughthese was, the magnetic fields .of these currents. inithe halves of "thecoilsPa-re opposed and balance out, thereby eliminating humand iron coresaturation. Owing'to the fact'th'at the-novel double impedance amplifierof v this invention servestoamplify the lower: frequencies bet"- terthan transformers, the cancelingout of the extraneous A. C. voltagesdelivered "by the B eli'minatorto theicen-te'r taps-of the plate choke'coils is an "important feature of the present invention. 1

Although the electrodes of the thermionic devices are illustrated'asssupplied with on to be understood that direct current supply may beused if desired. i T

Asvmany changes 1 could' be made the ergy from alternating.current-sources, it is above construction and many apparently widelyd'iflerent embodiments of this: vention could .be made without departingfromthe scopethereof, it is intended that all matter contained. in theabove" description or shown the; accompanying drawing shall beinterpreted as i-llu'strati-vewandnot :in a

limiting. sense. I 5

1. In an audio frequency amplifier circuit,- the combination withalpaiir of ele-c'tron'dis charge devices, each. of said devices having agrid element togetherjwitli acathode and anode of external: "circuitsextending. between the cathode and grid elements ofsa-id'de vice's,transformer: means having two magnetically coupled win-dings solidlyconnec tedin series, said transformerineans' having the'tfirst of itswindings connected ini-the externa circuit of one of said devices andthe second o-fit's windings connected in the external circuitof theother of said deviees and a pair-es eon pli ng" circuits, the firstren-a coupling circuits being connected to the first winding of saidtransform'einmeans, and the second of 'said coupling circuits beingconnected to. the

second winding of said transformer means, whereby current flowing insaid first coupling circuit through said first winding acts tosupplement thetvoltage in said second wind- -ing,-andcurrent flowing insaid second coupling circuit through said second winding acts tosupplement the voltage in said first winding, said first and secondwindings serving, respectively, as grid'chokes for the respectiveelectronv discharge devices. :2. .In an audio frequency amplifiercircuit, the:combination. with a pair of electron discharge devices,each of said devices having a grid elements together with a cathode andanode, of external circuits extending between the cathodeand gridelements of said devices, transformer means having two mag neticallycoupled windings solidly connected in series, said transformer meanshaving the first of its windings connected in the external circuitofone' of said devices and the second of its windings connected in theexternal circuit of the other of said devices, and a pair of couplingcircuits, each of said coupling circuits having a condenser therein, thefirst .of saidcOuplingcircuits being connected to the first winding ofsaid transformer means, and the second of said coupling circuits beingconnected to the second winding of said transformer means, whereby oneof said condensers and the first winding of said transformer means arearranged in series in said first coupling circuit and the other of saidcondensers and. the second winding of said transformer means arearranged in'series in said second coupling circuit.

' 3.111 a audio amplifier circuit, in combination, a pair of electrondischarge devices arranged in oppositionand adapted to be connected tosignal circuits, each of said devices having a grid element togetherwith a cathode and anode, external circuits extending between thecathode and grid elements of said devices, a center tapped transformermeans having portions thereof respectively included in said respectivecircuits, the ends of said transformer means being respectivelyconnected to the respective grid elements of said devices and its tappedcenter being connected to thecathodes of said devices, the respectiveportions of said transformer means serving as grid chokes for therespective discharge devices, a second pair of similar electrondischarge devices having external circuits extendingbetween the cathodeand grid elements of said devices, and a pair of condensers, each ofsaid condensers serving to couple the external circuit of a respectiveone of said second pair of electron discharge devices to the anode of. arespective one of said first pair of electron discharge devices.

4. In a push-pull double impedance coupling circuit for audioamplification, in combination, a'pair of thermionic tubes arranged inopposition and having cathode and grid elements, a pair of couplingcondensers, each of said condensers, respectively, serving to couple arespective one of the cathode grid circuits of said tubes to arespective input circuit, transformer means having two magneticallycoupled windings solidly connected in series, said transformer meanshaving the first of its windings connected in the external circuit ofone ofsaid tubes and the second of its windings connected in theexternal circuit of the other of said tubes, whereby current flowing inone of the input circuits through one of said condensers and said firstwinding, acts to supplement the voltage in said secondwinding, andcurrent flowing in the other input circuit through the other of saidcondensers and said second winding acts to supplement the voltage insaid first winding. v

In testimony, that I claim the invention set forth above I have hereuntoset my hand this 15th day of August, 1929.

EDWARD E. HILER.

